The present invention relates to the field of communications in general and more particularly, to determining the position of a mobile terminal device.
Wireless communication systems (networks) are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990's. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
It is desirable, and in certain places mandated by law, that mobile telecommunication network providers be able to determine an approximate geographical location of a mobile terminal (MT), such as, for example, an actively communicating cellular telephone.
A variety of MT location techniques have been proposed. These location techniques include uplink signal location, downlink signal location, Global Positioning System (GPS) based approaches, assisted GPS approaches combining communication signals and GPS signals and approaches based on digital television signals. For “uplink signal” location techniques, the mobile telecommunications network is typically configured to determine where the MT is located based on ranging measurements associated with one or more uplink signals. These uplink signals are transmitted by the MT and received by a number of receivers having known locations, such as, for example, cellular telephone base stations (BSs). For the “downlink signal” location techniques, the mobile telecommunications network is typically configured to determine where the MT is located based on ranging measurements associated with the reception, by the MT, of downlink signals from a number of transmitters having known locations.
FIG. 1 illustrates a conventional terrestrial mobile (wireless) telecommunications network 20 that may implement any one of a variety of known wireless communications standards including uplink and downlink signals. The wireless network may include one or more wireless mobile stations 22 that communicate with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular radiotelephone network may comprise hundreds of cells, and may include more than one MTSO 28 and may serve thousands of wireless mobile stations 22.
The cells 24 generally serve as nodes in the network 20, from which links are established between wireless mobile stations (terminals) 22 and a MTSO 28, by way of the base stations 26 servicing the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel that may be used for downlink transmission (network to mobile) of cell identification and paging information. The traffic channels carry the voice and data information. Through the network 20, a duplex (downlink and uplink) radio communication link 30 may be effected between two wireless mobile stations 22 or between a wireless mobile station 22 and a landline telephone user 32 via a public switched telephone network (PSTN) 34. The function of the base station 26 is commonly to handle the radio communications between the cell 24 and the wireless mobile station 22. In this capacity, the base station 26 functions chiefly as a relay station for data and voice signals. It is also know to provide mobile telecommunications networks in which the base stations are satellites, having associated coverage areas, rather than terrestrial base stations.
The GPS location approach generally uses location services not associated with either the uplink or downlink signals used in the mobile telecommunications network. In a typically GPS application, the GPS receivers collect and analyze ranging measurements from signals transmitted by GPS satellites having known locations.
As illustrated in FIG. 2, GPS is a space-based triangulation system using satellites 42 and GPS control computers 48 to measure positions anywhere on the earth. GPS was first developed by the United States Department of Defense as a navigational system. The advantages of this navigational system over land-based systems are that it is not limited in its coverage, it provides continuous 24-hour coverage, which may be highly accurate regardless of weather conditions. In operation, a constellation of 24 satellites 42 orbiting the earth continually emit a GPS radio signal 44. A GPS receiver 46, e.g., a hand-held radio receiver with a GPS processor, receives the radio signals from the closest satellites and measures the time that the radio signal takes to travel from the GPS satellites to the GPS receiver antenna. By multiplying the travel time by the speed of light, the GPS receiver can calculate a range for each satellite in view. Ephemeris information provided in the satellite radio signal typically describes the satellite's orbit and velocity, thereby generally enabling the GPS processor to calculate the position of the GPS receiver 46 through a process of triangulation. It is known to include a GPS receiver 46 in a mobile station 22 to provide position location functionality to the mobile station 22.
The startup of a GPS receiver typically requires the acquisition of a set of navigational parameters from the navigational data signals of four or more GPS satellites. This process of initializing a GPS receiver may often take several minutes. The duration of the GPS positioning process is directly dependent upon how much information a GPS receiver has initially. Most GPS receivers are programmed with almanac data, which coarsely describes the expected satellite positions for up to one year ahead. However, if the GPS receiver does not have some knowledge of its own approximate location, then the GPS receiver may not be able to find or acquire signals from the visible satellites quickly enough, and, therefore, cannot calculate its position quickly. Furthermore, it should be noted that a higher signal strength is typically needed for capturing the C/A Code and the navigation data at start-up than is needed for continued monitoring of an already-acquired signal. It should also be noted that the process of monitoring the GPS signal may be significantly affected by environmental factors. Thus, a GPS signal which may be easily acquired in the open typically becomes harder to acquire when a receiver is under foliage, in a vehicle or in a building.
Recent governmental mandates, e.g., the response time requirements of the FCC Phase II E-911 service, make it imperative that the position of a mobile handset be determined accurately and in an expedited manner. Thus, in order to implement a GPS receiver effectively within a mobile terminal while also meeting the demands for fast and accurate positioning, it has become desirable to be able to quickly provide mobile stations with accurate assistance data, e.g., local time and position estimates, satellite ephemeris and clock information, and visible satellite list (which generally varies with the location of the mobile station). The use of such assistance data can permit a GPS receiver that is integrated with or connected to a mobile station to expedite the completion of its start-up procedures. It is, therefore, desirable to be able to send the necessary GPS assistance information over an existing wireless network to a GPS receiver that is integrated with or connected to a mobile station.
Taylor et al., U.S. Pat. No. 4,445,118, discusses the concept of aiding or assisting GPS receivers. The method described uses a single transmitter, such as a geosynchronous satellite, to provide a single assistance message for a wide geographical area. The assistance message data includes a list of GPS satellites in view, the respective satellite positions, and predicted Doppler shifts on the satellite signals. This structure of this message permits the position computation function (PCF) to be done in the user receiver.
Krasner, U.S. Pat. No. 5,663,734, describes another GPS receiver approach. The patent is mainly related to the receiver architecture, but discusses how the receiver performance can be improved by assistance. The patent mentions “data representative of ephemeris” and expected Doppler shifts as possible contents of the assistance message.
Lau, U.S. Pat. No. 5,418,538, describes a system and method for aiding a remote GPS/GLONASS receiver by broadcasting “differential” information from a like receiver in a “reference station.” The reference station broadcasts a visible satellite list and also the associated ephemeris, in one embodiment. The advantages to the remote receiver may be three-fold: reduced memory requirements, lower-cost frequency reference, and faster acquisition. The discussion describes the benefit of being able to estimate and remove the Doppler due to the receiver clock inaccuracy after acquiring the first satellite.
Eshenbach, U.S. Pat. No. 5,663,735, describes a method whereby a GPS receiver derives an accurate absolute time reference from a radio signal. Optionally, the receiver also derives from the radio signal a frequency reference that is more accurate than the inexpensive crystal oscillator contained in the receiver. The GPS receiver performs the position calculation, and therefore must have the absolute time as well as the ephemeris and clock corrections for the GPS satellites.
Another assisted-GPS standard for GSM-based networks is described in specification numbers 3GPP TS 04.31 and 3GPP TS 03.71. This standard is based on placing reference GPS receivers at various nodes in the network, capturing the ephemeris information from these receivers, then providing this information along with a list of visible satellites to all handset-based GPS receivers via messages on GSM downlink bearers. The benefit of this approach is that it allows the handset-based GPS receiver to be fully functional, i.e., it contains the PCF and also can operate in continuous navigation mode.
One particularly challenging, but important, component for which assistance would be beneficial is obtaining accurate GPS timing information at the GPS receiver. Traditionally, GPS receivers demodulate the required timing information from the messages broadcast by the GPS satellites. However, reasonably error free demodulation of such signals may not be possible below a certain signal threshold, which itself may be significantly higher than the minimum signal level required for tracking already acquired signals and making range measurements. Accordingly, where GPS receiver operation is desirable under conditions of low-signal operation (for example, due to environmental attenuation, antenna compromises or other affects) it may not be possible to rely on demodulation of the transmitted information from the GPS satellites as a source of GPS timing information.
As noted above, one previously proposed approach is the provision of assistance information from a cellular network to the combined GPS and cellular receiver. Three different techniques for providing such GPS timing information through network assistance have previously been proposed. First, some networks are synchronized by GPS. An example is the IS-95 Code Divisional Multiple Access (CDMA) system that, as a result, has an implicit timing relationship between the air-interface timing (i.e. the spreading codes) of the communication network and GPS timing. Therefore, once a GPS-equipped mobile terminal (GPS-MT) synchronizes with the communication network air-interface it is expected to also have accurate GPS timing that can be used to improve the sensitivity and time-to-first-fix (TTFF) of the GPS receiver in the device. This approach is only useful, however, for communication networks, such as IS-95 CDMA, which have such an implicit timing relationship.
One approach proposed for networks that are not so GPS-synchronized is to establish a relationship between GPS timing and a communication network's air-interface timing at each cell transmitter (base station) of the communication network by provision of an observer unit equipped with a GPS receiver as well as a cellular receiver. This timing relationship information can then be reported to a GPS assistance server of the communication network and thereby included in assistance messages sent to GPS-MT devices being serviced by the respective base station of the communication network. Accordingly, once a GPS-MT device synchronizes with the air-interface timing of its serving cell of the communication network and receives this timing assistance, it may determine the current GPS timing accurately. Systems incorporating this second approach are described in U.S. Pat. No. 6,240,808 entitled Method and System for Aiding GPS Receivers Via a Cellular or PCS Network.
A third approach that may be applied to unsynchronized networks without a GPS observer unit (also referred to as a Location Measurement Unit (LMU)) at each base station location is described in U.S. Pat. No. 5,812,087 entitled Method and Apparatus for Satellite Positioning System Based Time Measurement. In this approach, the timing information is derived from samples of the navigation signal from multiple GPS satellites. For example, GPS-MT device may make measurements on the ranging codes of multiple GPS satellite signals and also sample some duration of the navigation data that is imposed on these codes. This data may then be returned to a server where the navigation data samples may be matched to the samples of a reference signal to estimate the time at which the other measurements were made.
Another approach to a reduced complexity GPS location service to satisfy governmental mandates for FCC Phase II E-911 service provides only a simplified, GPS receiver in the MT, rather than a full function autonomous GPS receiver. An assisted location service 36 (FIG. 1) associated with the communication network then is used to calculate the position of the MT. Such an approach is specified in the TIA/EIA/IS-801-1 specification (IS-801), which is implemented in the GPSOne protocol assisted location service available from SnapTrack Inc, a Qualcomm Company, as described at the website http://www.snaptrack.com.
A GPSOne compatible receiver (i.e. located in the mobile terminal) generally performs all GPS satellite acquisition functions and then sends measurements to a centralized location server of a CDMA network serving the mobile terminal. The raw measurements, as specified by IS-801, include code phase, measurement time and signal quality parameters. By generating only intermediate navigation data at the GPSOne compatible receiver, some of the burden of performing positioning calculations may be shifter to the location server. Thus, a GPSOne compatible receiver is structured to output intermediate raw measurements as contrasted with a full function autonomous GPS receiver that generally does not output such intermediate raw measurements.
While such a reduced capability receiver may be satisfactory for meeting E-911 requirements to provide position location information for a MT that are accessible at the communications network, a GPSOne type receiver typically does not provide the actual position information at the mobile terminal. Such positione information would generally need to be computed at the location server and then be transmitted back to the MT. The round-trip delay for such an approach to providing positioning information would generally not meet location application requirements for rapid access to repetitive position fixes, a capability which is supported by autonomous full function GPS receivers coupled to the MT. One approach to such systems would be to provide both an IS-801 compatible receiver to meet the E-911 position requirements along with a distinct, autonomous GPS receiver that provides rapid access to repetitive position fixes.